The invention relates to an apparatus for cooling and calibrating an extruded plastic profile with a cooling tank which receives a cooling liquid and can be charged with a pressure below atmospheric, and with at least one calibrating device arranged within the cooling tank for the plastic profile pulled through the cooling tank, with the calibrating device being composed of individual parts extending in the direction of passage of the plastic profile.
During the extrusion of plastic profiles, the hot profile strand emerging from the shaping extrusion die is calibrated and cooled, with a cooling tank being provided for cooling through which the plastic profile is pulled with the help of a draw-off device provided downstream of the cooling tank. The plastic profile is treated within the cooling tank with a cooling liquid, which is generally water. The dimensional accuracy is thus ensured by calibrating elements through which the plastic profile is guided within the cooling tank. Said calibrating elements which are adjusted to the cross-sectional shape of the plastic profile are arranged on the inlet side of the cooling tank at a small distance from each other, which distance increases gradually towards the outlet side of the cooling tank. As a result of these increasing distances in the direction of passage between the successive calibrating elements it is intended to limit the number of calibrating elements to a number which is necessary for securing the dimensional accuracy. Notice must be taken in this connection that the strength and the ability to hold the shape is still very low in the inlet zone of the cooling tank due to the only gradually progressing solidification progressing from the outside to the inside, so that the plastic profile needs to be supported with low axial distance by the calibrating elements in order to avoid any bulging of the profile walls in the zone of hollow chambers between the successive calibrating elements due to the usual pressurization of the cooling tank with pressure below atmospheric. Despite the enlargement of the mutual distances of the calibrating elements which is possible depending on the solidification-induced increase in the dimensional stability of the plastic profile, the required number of calibrating elements remains high, leading to a high production effort for such calibrating devices. When setting the calibrating device which consists of a plurality of calibrating elements, the profile elements are subsequently machined in several tests on the basis of deviations of the profile cross section from the setpoint cross section which are determined in said tests, with the amount of work for such subsequent machining increasing with the number of used calibrating elements. This applies especially when the cross section of the profile elements needs to be reduced in sections, which then frequently requires displacements of individual parts of the calibrating elements with respect to each other, the provision of additional inserts and a partly new production of calibrating elements.
Calibrating devices are also used instead of calibrating elements which form a cooling block which can be charged with a cooling liquid with a continuous calibrating opening, through which the plastic profile is drawn. Vacuum slots opening into the zone of the calibrating opening ensure that the plastic profile rests on the wall of the calibrating opening. In order to simplify the production of such calibrating devices it is known (DE 28 09 386 A1, DE 25 35 286 A1) to construct the cooling blocks from individual parts which extend in the direction of passage of the plastic profile and are screwed together into a cooling block forming the calibrating opening. These known calibrating devices are hardly suitable for a direct charging of the plastic profile with cooling liquid.
The invention is thus based on the object of providing an apparatus for cooling and calibrating an extruded plastic profile of the kind mentioned above in such a way that the calibrating device can be set up in a considerably easier way for a plastic profile due to a reduction in the amount of work required for construction.
This object is achieved by the invention in such a way that the parts of the calibrating device form calibrating elements which are distributed over the circumference of the plastic profile by leaving out gaps for the passage of cooling liquid.
The invention is based on the finding that for calibrating an extruded plastic profile in a cooling tank pressurized with a pressure below atmospheric it is not necessary to provide a circumferentially enclosed support of the plastic profile in the event that a support is ensured in such circumferential areas which tend to arch due to the pressurization with pressure below atmospheric. This is essentially only the case in the middle zone of profile walls delimiting existing hollow chambers, so that the conventional, circumferentially enclosed calibrating elements can be replaced by calibrating elements which extend along the calibrating device in the direction of passage of the plastic profile and are distributed over the circumference of the plastic profile by leaving out gaps for the passage of cooling liquid. The gap resulting between the calibrating elements ensures a direct cooling of the plastic profile by the cooling liquid, i.e. in such regions which have a higher cooling requirement due to corner formations or wall connections. The cooling via the calibrating elements of the middle regions of the profile walls which rest on the calibrating elements and delimit existing hollow chambers is thus sufficient, in cooperation with the liquid cooling between the calibrating elements, for a thorough solidification of the plastic profile during the passage through the calibrating device. Since the profile cross section to be calibrated is determined through the calibrating elements that continue over the calibrating device, it is merely necessary to machine the calibrating elements for the purpose of adjustment of the calibrating device to predetermined conditions, as a result of which the efforts for adjusting the calibrating device are substantially reduced. Moreover, the resistance to the passage of the plastic profile by the calibrating device can be reduced in comparison with calibrating devices made up of individual calibrating elements.
The fixing of the calibrating elements in their assignment determining the profile cross section can be solved constructionally in different ways. Particularly simple constructional conditions are obtained, however, when the calibrating elements are fastened detachably in at least two frames arranged transversally to the direction of passage, so that the calibrating elements form in combination with the frame a sufficiently dimensionally stable, self-supporting frame without obstructing the machining of individual calibrating elements which can be removed for this purpose from the frame.
As has already been explained, it can be assumed that the direct cooling of the plastic profile via the close-lying calibrating elements provides favorable cooling conditions in many cases in cooperation with the direct cooling by the cooling liquid. If the cooling performance needs to be increased, at least individual cooling elements can form cooling channels extending transversally to the direction of passage at the sides facing the plastic profile, so that the cooling is increased in the zone of the calibrating elements. If the cooling channels extend in an inclined manner with respect to the direction of passage, a conveying effect is obtained on the cooling liquid in the zone of the cooling channels by the plastic profile pulled through the calibrating device, leading to a further improvement of the cooling. The indirect cooling via the calibrating elements can be increased in such a way that at least individual calibrating elements are provided with longitudinal bores which are charged with cooling liquid, thus ensuring a favorable discharge of the heat absorbed from the cooling elements. Said longitudinal bores of the calibrating elements which are charged with cooling liquid can be additionally in connection with the cooling channels of the calibrating elements in order to bring about a forced circulation of the cooling liquid in the zone of said cooling channels.